In a 3GPP (Third Generation Partnership Project) TDD (Time Division Duplex) system, a receiver utilizes channel estimation codes (midamble codes) and responses to channel estimation codes from channel estimation windows to measure the Interference Signal Code Power (ISCP). A response result from the channel estimation windows includes taps with signal response components and taps without signal response components; wherein, the tap without signal response component represents a response to the interference and is referred as an interference response tap. The interference signal power (or referred as interference signal code power) can be obtained through measurement and statistic of these interference response taps.
In practical applications, usually either of the following two methods is used to obtain the interference response and the ISCP:
The first method is used to measure the interference power by using idle channel estimation windows. During the channel estimation, each of the channel estimation codes obtained by shifting the basic midamble codes corresponds to each channel estimation window. The total length of the channel estimation windows P remains constant (e.g., P=128). In a time slot, if one or more channel estimation codes are not used to send signals, then the channel estimation window(s) corresponding to the channel estimation code(s) is referred as idle channel estimation window(s). In the original channel response results, the idle channel estimation windows correspond to responses to interference signals. The interference power σn2 can be measured with these responses from the idle channel estimation windows:
                              σ          n          2                =                              P                          D              ·              W                                ⁢                      1                          (                                                                                          #                      ⁢                                                                                          ⁢                      of                      ⁢                                                                                          ⁢                      all                      ⁢                                                                                          ⁢                                              k                        m                                                                                                                                                        not                      ⁢                                                                                          ⁢                      used                                                                                  )                                ⁢                                    ∑                                                                                          all                      ⁢                                                                                          ⁢                                              k                        m                                                                                                                                                        not                      ⁢                                                                                          ⁢                      used                                                                                            ⁢                                          ∑                                  i                  =                  0                                                  W                  -                  1                                            ⁢                                                                                    h                    _                                    i                                      (                                          k                      m                                        )                                                                                                                          (        1        )            
wherein, W represents the length of a channel estimation window. First, the total power of window km (sum of W tap powers) is calculated with the channel estimation result hi(km); then, the total power of all idle channel estimation windows (sum of total powers of all idle channel estimation windows) is obtained (wherein, km represents a specific idle window (all km not used)); next, the average of all taps from all idle channel estimation windows, i.e., the interference power, is obtained by multiplying the resulting value by P and then dividing by D and W and the number of all idle channel estimation windows (# of all km not used). Wherein D is the noise degradation factor and greater than 1, corresponding to the basic midamble codes (a complex value sequence mi, i=1 . . . P), and can be obtained with the following equation:
                    D        =                                            ∑                              l                =                1                            P                        ⁢                                                                            1                                                            ∑                                              i                        =                        1                                            P                                        ⁢                                                                  m                        i                                            ⁢                                              ⅇ                                                                              -                            j2π                                                    ⁢                                                                                                          ⁢                                                      li                            /                            P                                                                                                                                                                          2                                >          1                                    (        2        )            
Performing interference power estimation with the method expressed by equations (1) and (2) has an advantage in performance; however, the premise is: idle channel estimation windows are required and the positions of the idle channel estimation windows must be available. The idle channel estimation windows usually exist; however, there are difficulties in obtaining positions of the idle channel estimation windows:
For uplinks, since the channel estimation windows are assigned by a base station, the base station knows the positions of the idle channel estimation windows; therefore, the above method is advantageous in estimation of interference power for uplinks;
However, for downlinks, a user terminal can obtain the positions of estimation windows in any of the following three ways:                1) defining a special channel estimation window in each time slot and keeping it in idle status permanently or at a position of a certain frame cycle;        2) sending the idle channel estimation window information to the user terminals by base station broadcasting or signaling;        3) for the active time slot, keeping the channel estimation window of the user in idle status in a specific frame.        
The above method of obtaining idle channel estimation window information for downlinks will cause waste of system resource, complexity in implementation, and real-time performance. Viewed from the current situation, it is impossible for the existing mobile communication standards to support user terminals to obtain idle channel estimation window information; therefore, for user terminal applications, it is impossible to perform measurement of interference power with the idle channel estimation window method.
The second method is used to perform measurement of interference power by post-processing against SNR (Signal-to-noise Ratio) threshold. Treating the channel response estimation result against SNR threshold can separate signal response from interference response. In detail, the post-processing against SNR threshold includes: determining whether the SNR of each tap in the original channel estimation result is higher than a specific threshold; if it is higher than the specific threshold, the channel estimation value at the tap is deemed as the signal response; if it is lower than the specific threshold, the channel estimation value at the tap is deemed as the interference response. Suppose the specific SNR threshold is εCHE and the reference threshold of interference power is σn2, then the corresponding power threshold is:ΓCHE=σn2εCHE/P  (3)After post-processing, the interference response is:
                                          h            _                    i          ″                =                  {                                                                                                                h                      _                                        i                                    ;                                                                                                                                                                                                                              h                            _                                                    i                                                                                            2                                        <                                          Γ                      CHE                                                        ,                                      i                    =                                          1                      ⁢                      …                      ⁢                                                                                          ⁢                      P                                                                                                                                            0                  ;                                                            else                                                                        (        4        )            
The above equation indicates: if the channel response is lower than the interference power threshold, it is kept; otherwise it is discarded (i.e., zero). Suppose the number of taps of interference response after post-processing is Wn, then the interference power σn2 can be calculated with the following equation:
                              σ          n          2                =                              P                          D              ·                              W                n                                              ⁢                                    ∑                              i                =                1                            P                        ⁢                                                                                                h                    _                                    i                  ″                                                            2                                                          (        5        )            
Wherein, D is the noise degradation factor of corresponding basic midamble code and is calculated with equation (2).
Performing estimation of interference power with this method has an advantage in performance; however, the premise is: the reference threshold of interference power, i.e., σn2 in equation (3), must be available; but what is solved in equation (5) is just the interference power σn2. If the priori value is unavailable or the error of the priori value is high, performing estimation of interference power with the method makes no sense. Therefore, in actual measurement of interference power, the method can't be used separately, unless it is used in a situation without idle channel estimation windows in conjunction with the idle channel estimation window method.
In conclusion, in 3GPP TDD systems, the existing ISCP measuring methods can't meet the requirements in downlink terminal applications. However, for normal system operation, it is required that the user terminals can measure the ISCP in active time slots and non-active time slots in downlinks; therefore, an effective method for measuring ISCP of user terminals in downlinks is required.